1. Field of the Invention
The present invention relates to a capacitance-type displacement detector and measuring apparatus adaptive for a small-sized measuring device such as an electronic micrometer, hole test, angle gauge and slide caliper.
2. Description of the Related Art
A capacitance-type displacement detector with a low power-consumption suitable for downsizing is utilized in a small measuring device such as a slide caliper and semi-cylindrical capacitance-type rotary detector. Such the capacitance-type displacement detector is configured to move two scales relatively to each other in order to measure an amount of a relative movement between both scales by detecting an electrostatic capacitance variation between electrodes arranged on these scales.
FIGS. 9 and 10 show a rough arrangement of a conventional capacitance-type displacement detector. The detector comprises a first scale 1 and a second scale 2 that are arranged so that they can move relatively to each other, opposing to each other and interposing a certain gap therebetween. Transmitting electrodes 3 and a receiving electrode 4 are formed on the first scale 1. The transmitting electrodes 3 are disposed with a predetermined pitch in the direction of a displacement to be detected. In this example, eight transmitting electrodes define one transmitting electrode unit that corresponds to a basic period (W1), and four such the units U1-U4 define a transmitting electrode array. The receiving electrode 4 is disposed adjacent to the transmitting electrodes 3 in the direction perpendicular to the displacement direction. The receiving electrode 4 comprises a single electrode with a width of L2 shorter than a width of L1 of the transmitting electrode group. Further definitely, the width L2 of the receiving electrode 4 is shorter than the width L1 of the transmitting electrodes 3 by a width of W1 of one transmitting electrode group unit (equal to eight transmitting electrodes 3). Thus, each of both ends of the receiving electrode is located at an inner position by a distance of four transmitting electrodes (W1/2) from respective both ends of the transmitting electrode group.
Formed on the second scale 2 are coupling electrodes 5 and ground electrodes 6 that capacitively couple to the transmitting electrodes 3 and receiving electrode 4. An arrangement period of the coupling electrode 5 and ground electrode 6 in the displacement direction is coincident with the width of one unit of the transmitting electrodes 3, W1, that is the basic period. Widths of the coupling electrode 5 and ground electrode 6 are set to L/3 that is equal to about a half the basic period.
Supplied to each unit of the transmitting electrodes 3 are eight-phase modulation pulse signals with a 45.degree. out of phase between every two signals output from a pulse modulator circuit 7. A total phase of the modulation pulses received at the coupling electrodes 5 varies in accordance with an amount of a relative displacement in the scale displacement direction between one unit of the transmitting electrodes 3 and the coupling electrodes. Phase information of the modulation pulses received at the coupling electrodes 5 is directly transferred to the receiving electrode 4. The phase information received at the receiving electrode 4 is processed at a measurement circuit 8 in order to obtain an amount of a relative displacement between the first scale 1 and the second scale 2.
As described above, the width L2 of the receiving electrode 4 is set shorter than the width L1 of the transmitting electrode group formed on the first scale 1 in such the capacitance-type displacement detector. This is because capacitive couplings of the transmitting electrode group at both ends with the coupling electrodes 5 are partial and therefore received phases at two receiving electrodes 4 that couple both ends of the transmitting electrode group are discordant with each other. If the width L2 of the receiving electrode 4 is equal to the width L1 of the transmitting electrode group, the discordant phases at both ends may badly affect an accuracy in detecting the amount of the displacement. For this reason, the both ends of the receiving electrode 4 are usually cut off by the width L3 of the coupling electrode 5, respectively, whereby the width L2 of the receiving electrode 4 becomes shorter than the width L1 of the transmitting electrode group. Thus, the receiving electrode 4 may capacitively couple only with the coupling electrodes 5 that exhibit the same received phases for the amount of the scale displacement.
In the above described conventional capacitance-type displacement detector, however, since the transmitting electrode group and receiving electrode 4 have different widths L1 and L2, noise components mixed into the receiving electrode 4 directly from the transmitting electrodes 3 without bypassing the coupling electrodes 5 become unbalanced, affecting a measurement accuracy. FIG. 11 is a diagram, for use in explanation of this matter, which shows enlarged electrode patterns of the transmitting electrodes 3 and receiving electrode 4. Noises with respective phase components .DELTA.0.degree., .DELTA.45.degree., . . . , .DELTA.315.degree. from respective transmitting electrodes 3 are mixed into the receiving electrode 4. In addition to the noises from transmitting electrodes 3a that are located within the width L2 of the receiving electrode 4, noises .delta.135.degree. and .delta.180.degree. from transmitting electrodes 3b and 3c that are located outside the width and close to both ends of the receiving electrode 4 may also be mixed into the ends. A vector diagram in FIG. 12A shows these noise components mixed in the receiving electrode 4. FIG. 12B shows a total vector of these noises. If the width L1 of the transmitting electrode group is equal to the width L2 of the receiving electrode 4, the mixed noises .DELTA.0.degree.-.DELTA.315.degree. are cancelled to zero. If L1&gt;L2, however, the presence of the transmitting electrodes 3b and 3c located outside the width L2 of the receiving electrode 4 may generate the mixed noises .delta.135.degree. and .delta.180.degree. which become noise components to be mixed into a measurement value, resulting in a degradation of the S/N ratio.
In order to solve this problem, several methods are employed to improve the S/N ratio in the art, such as:
(1) Increasing the number of the units of the transmitting electrodes 3; PA0 (2) Extending a distance between the transmitting electrodes and the receiving electrode; and PA0 (3) Providing ground electrodes for shielding between the transmitting electrodes and the receiving electrode.
In case of further downsizing the measuring device, however, each of the above methods (1)-(3) may prevent the downsizing. Even if downsizing the measuring device by reducing the number of the units and shortening the distance between the electrodes, therefore, a method capable of effectively reducing the mixed noises between the electrodes has been desired.